Abstract The base-catalyzed rearrangement of the esters of 2H-1, 2-benzothiazolin-3-one-2- acetate 1, 1-dioxide (IIa-IIc) afforded substituted 2H-1, 2-benzothiazine-3-carboxylate 1, 1- dioxides IIIa-IIIc. After N-alkylation and aminolysis the antiinflammatory drugs piroxicam and isoxicam were obtained.